[ruby-core:122744] [Ruby Feature#21346] Introduce `String#ensure_suffix`

Issue #21346 has been updated by matheusrich (Matheus Richard).


I've updated the documentation examples to reflect the typical use:

```rb
"script".ensure_suffix(".rb")   # => "script.rb"
"script.rb".ensure_suffix(".rb") # => "script.rb"
"script.r".ensure_suffix(".rb")  # => "script.r.rb"
```

----------------------------------------
Feature #21346: Introduce `String#ensure_suffix`
https://bugs.ruby-lang.org/issues/21346#change-114024

* Author: matheusrich (Matheus Richard)
* Status: Open
----------------------------------------

## Problem

Ensuring a string has a specific suffix or prefix is a common operation in many applications.
Bundler itself uses it:

https://github.com/rubygems/rubygems/blob/d409ec8b5fc647fabe30e37e17cd1ea857634f6b/bundler/lib/bundler/uri_normalizer.rb#L17

Here are [GitHub search](https://github.com/search) queries that might find this pattern in other places:
1. for Ruby: `/end(?:s)?_with\?\(['"].*['"]\) \?/ lang:ruby -is:fork`
1. for Crystal (a language very similar to Ruby): `/ends_with\?\(['"].*['"]\) \?/ lang:crystal -is:fork`

## Suggested solution

I believe Ruby would benefit from having a first-class method for this purpose.
I suggest the `String#ensure_suffix` and `String#ensure_prefix` methods.

I think these names are intuitive enough (here are 2 examples of people using `ensure` for this purpose ([1](https://github.com/boltops-tools/ufo/blob/796104fdb89163d09a58fad42add697923c18294/lib/ufo/cfn/stack/builder/resources/dns.rb#L76), [2](https://github.com/mumuki/mumuki-domain/blob/6194089d82b1a0c8805ecba98e006deb1694dc8e/lib/mumuki/domain/extensions/string.rb#L6))).

I've gone ahead and implemented `String#ensure_suffix` in a [pull request](https://github.com/ruby/ruby/pull/13366) but the suggested behavior is this:

```rb
"Hell".ensure_suffix("o!")   # => "Hello!"
"Hello!".ensure_suffix("o!") # => "Hello!"

s = "Hello!"
s.ensure_suffix("!").equal?(s) # => true # returns same object if already suffixed
```





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